Broadly speaking, polyimide films are known for use as base films in flexible printed circuit boards (“FPCs”). As FPC technology progresses however, certain film property improvements become increasingly desirable or necessary.
One important such film property is the rate of moisture vapor transmission. If this rate is unduly low, film processing can become difficult (if not impossible or impractical) due to unwanted surface blistering of the film, otherwise known in the industry as “popcorning”.
In addition, a need also exists for polyimide films having improved strength properties, e.g., tensile modulus. Although polymer strength is often improved by stretching (orienting) a polymer above the polymer's glass transition temperature (“Tg”), many high performance polyimide polymers for FPC applications have a Tg so close to the decomposition temperature of the polymer, that such heat-stretch orientation of the polymer film is not practical or even possible. Hence new ways for increasing strength properties are needed.
Unfortunately, as a polymer's strength properties are increased, oftentimes the polymer's rate of moisture vapor transmission will unduly decrease. Nevertheless, the FPC industry is demanding further increases in strength properties for polyimide films, while also demanding acceptable rates of moisture vapor transmission, as well as acceptable etch rates, dimensional stability and processability.
Preferred embodiments of the present invention can be manufactured in accordance with the polyimide processes and methods taught in:                U.S. Pat. No. 5,166,308, “Copolyimide Film With Improved Properties,” Assigned to: E. I. Du Pont de Nemours and Company, Inventors: John A. Kreuz and Richard F. Sutton, Jr. application Ser. No. 516,887, Filed Apr. 30, 1990, Issued Nov. 24, 1992,which is hereby incorporated into this specification by reference for all teachings therein.        